Allow Docker engines to fetch Anubis private key from Vault

Summary:
Generate a different key on each machine, so we don't need to update
Vault policy each time we add a new service.

Ref T2193.

Reviewers: ptdradmin, Duranzed

Reviewed By: Duranzed

Maniphest Tasks: T2193

Differential Revision: https://devcentral.nasqueron.org/D4015